1. Field of the Invention
The invention relates to a process for melting and refining vitrifiable materials for the purpose of continuously feeding glass-forming plants with molten glass.
2. Description of the Related Art
More particularly intended are plants for forming flat glass such as float or rolling plants, but also plants for forming glassware of the bottle or flask type, plants for forming glass fibres of the mineral wool type for thermal or acoustic insulation or else textile glass fibres called reinforcing fibres.
A great deal of research has been carried out on these processes, which schematically comprise a first melting step followed by a refining step intended to condition the molten glass thermally and chemically and in eliminating therefrom any batch stone, bubbles or any cause of defects appearing after forming.
In the melting range, it has thus been sought, for example, to speed up the melting process or to improve its energy efficiency. Mention may thus be made of the process consisting in rapidly heating the vitrifiable materials in a homogeneous and controlled manner while carrying out intense mechanical stirring allowing the still-solid vitrifiable materials to be brought into intimate contact with the already-liquid phase. This process is especially detailed in Patents FR-2,423,452, FR-2,281,902, FR-2,340,911 and FR-2,551,746 and generally uses electrical heating means of the submerged-electrode type.
Another type of melting process has been developed, for example of the type of those described in U.S. Pat. No. 3,627,504, U.S. Pat. No. 3,260,587 or U.S. Pat. No. 4,539,034 which consist in using, as heating means, submerged burners, that is to say burners fed with gas and air, these generally being placed so as to be flush with the bottom wall so that the flame develops within the mass of vitrifiable materials during liquefaction.
In either case, although it is possible actually to very significantly reduce the residence time of the vitrifiable materials in the melting chamber and to considerably increase the production efficiency compared with “conventional” melting operations, the molten glass being molten is, on the other hand, in the form of a foam which is difficult to refine—it is especially difficult to guarantee the quality of the final glass, especially optical glass.
Research has also been conducted in the refining field. Thus, it is, for example, known from Patent EP-775,671 and U.S. Pat. No. 4,919,097 to carry out at least part of the refining operation under reduced pressure, thereby making it possible, for example, to obtain glass very low in sulphates and with a high redox. However, such refining causes intense foaming, which may be difficult to control and remove.